In the case of in-circuit trimming applications, a typical goal is to trim one or more resistors which are part of the circuit to adjust an overall circuit output which depends on those resistances. This circuit output may be any measurable quantity, but is typically translated into a voltage or current to be measured electrically. For simplicity, let us refer to such a circuit output as “Vout”. Usually, it is more convenient not to directly measure the resistance values of those trimmable resistors, because this may require extra electrical connections and/or extra measurements. Instead, one understands the relationship between changes in the trimmable resistors (ΔR1, ΔR2, etc), and changes in the circuit output (ΔVout), or at least the relationship between the direction (sign) of each ΔR and the direction (sign) of changes in Vout. In this way, one can apply trims to the trimmable resistors, while monitoring only the intended Vout. This type of scenario applies to any trimming method (manual trimpot, laser trimming, digital trimming, thermal trimming).
Trimming methods exhibit differing properties and constraints of operation. Manual trimpots are fully bidirectional over their entire range of resistance, but require manual operation, may have limited precision, and may be prone to mechanical instability. Digital trimpots also may be fully bidirectional over their entire range of resistance, but are not passive resistors. Laser trimming has a more limited trim range, more limited bi-directionality, and is difficult to do after packaging.
Thermally-trimmable resistors feature electrically-driven trimming, may be done at any practical stage in the manufacturer-to-user chain (including after packaging), and once trimmed, they are purely-passive components. However, they may also have constraints on bi-directionality of trimming. These types of resistors may often be more easily trimmable in one direction than another. For example, certain thermally-trimmable polysilicon resistors may be readily trimmed down (in the direction of decreasing resistance), from its as-manufactured resistance value (Ras-mfr), by tens of percent of Ras-mfr, but after such a trim-down, may have only limited trim-up (recovery) range. Other trimming characteristics are also possible.
In an in-circuit trimming application, limitations to full bi-directionality significantly constrain what can be done, and/or constrain trimming performance (e.g. speed, precision, range). For example, if a single trimmable resistor (any trimming method), is trimmable only in one direction, then one must be more conservative in seeking the Vout target. One must approach more slowly, to make sure that circuit settling times are accounted for, and one must be mindful of the quantization of the resistance trim, since the next trim may jump too far. Another example, if more than one trimmable resistor is used, then the optimal position of one resistor may depend on the position of the other, and vice-versa, but trims must be done sequentially, and so an individual trim of one resistor may reach an interim (non-optimal) target while irreversibly overshooting the optimal target (which may not be known until another resistor (or resistors) has reached its optimal position, or close to its optimal position).
In the specific case of thermal trimming, if one doesn't know the final (optimal) trim target, it may be necessary for the adaptive trim to proceed more slowly, consuming more trimming time.